Lamp envelopes that have high operating temperatures are commonly made of a high melting point glass having a high silica content. Such glasses can be quartz, fused silica, Vycor (a glass having approximately 96 percent silica) and the like. Such lamp envelopes are usually sealed by a press seal at one or both ends thereof. The press seal is formed by heating the open end of a tubular lamp envelope to the softening point of the glass and pressing the softened end between a pair of jaws.
Examples of lamps having press seals of the type to which this invention relates are tungsten halogen lamps. The envelopes of such lamps utilize press seals in order to prevent seal failures due either to thermal expansion of the glass or of the metal conductor embedded therein. The high operating temperature of the lamps obviates the use of stem seals, such as are generally used in incandescent and fluorescent lamps, which have much lower operating temperatures.
It is known in the art of lamp envelopes having press seals, and in particularly those lamps of the tungsten halogen variety, to employ a thin refractory metal conductive foil to establish an electrical connection between the lamp's external (outer) lead wires and the internal filament structure (or electrodes). Several electric lamps possessing such components are available on the market today. Typically, such foils are produced from thin molybdenum stock (for its ductility).
As further described below, the present invention relates particularly to the electrical connection between such a conductive foil member and the lamp's internally positioned filament structure. With greater particularity, the invention concerns the provision of a sound electrical connection between such a conductive foil member and a side rod support wire which forms part of such a filament structure. For definition purposes, the term side rod support wire is meant to include any internal conductive lead wire which projects within the lamp's envelope and is electrically coupled to and/or forms part of the lamp's filament structure. Such a wire also provides support for the filament(s) to thereby assure their proper orientation within the envelope.
Heretofore, connection between the side rod support wire (e.g., molybdenum) and conductive foil has typically involved a welding operation wherein a flux was required in order to obtain a relatively strong bond between both elements. This procedure has typically involved either utilization of a flux of substantially pure platinum, which understandably adds appreciably to the cost of the end product, or use of a less expensive flux (e.g., one consisting of platinum clad molybdenum foil or wire wherein the platinum comprised about fifteen percent by weight of the flux). This latter technique also possesses several disadvantages, however, in that it has typically required difficult to control manual operations (e.g., flux and foil orientation) as well as a double weld wherein the flux is first attached (to the side rod) and this assembly (flux and side rod) is thereafter welded to the foil. Such a double weld in the same area may cause side rod embrittlement and/or breakage. Another problem of this latter technique is that use of the heretofore cylindrical side rod results in only a single line of contact between side rod and foil, such that a weld along this location has occasionally markedly imprinted the molybdenum foil and caused it to tear. Still another problem when using a fifteen percent platinum flux has been the observation of platinum migration during welding to the outer regions of the junction, leaving between the two parts being joined a relatively thin amount of molybdenum with relatively little platinum intermixed therein. Effective weld control has thus been difficult to attain. Lastly, the aforementioned welding techniques have typically mandated relatively high current densities in order to provide a sound connection between the cylindrical support wire and flat foil. In one example, densities in excess of about 600 amperes per square millimeter have been required.
It is believed, therefore, that an electric lamp and method of making same wherein the aforementioned several disadvantages are overcome would constitute a significant advancement in the art.